Readout method and apparatus for optical information medium

ABSTRACT

An information readout method for an optical information medium comprising an information recording layer having pits or recorded marks representative of information data involves the step of irradiating a laser beam to the information recording layer through an objective lens for providing readings of the pits or recorded marks. When the laser beam has a wavelength λ of 400 to 410 nm, the objective lens has a numerical aperture NA of 0.70 to 0.85, and the pits or recorded marks have a minimum size P L  of up to 0.36λ/NA, readout is carried out at a power Pr of at least 0.4 mW for the laser beam. When the laser beam has a wavelength λ of 630 to 670 nm, the objective lens has a numerical aperture NA of 0.60 to 0.65, and the pits or recorded marks have a minimum size P L  of up to 0.36λ/NA, readout is carried out at a power Pr of at least 1.0 mW for the laser beam. Pits or recorded marks of a size approximate to the resolution limit determined by diffraction can be read out at a high C/N.

[0001] This invention relates to a method and apparatus for reading outinformation in an optical information medium.

BACKGROUND OF THE INVENTION

[0002] Optical information media include read-only optical discs such ascompact discs, rewritable optical recording discs such asmagneto-optical recording discs and phase change optical recordingdiscs, and write-once optical recording discs using organic dyes as therecording material.

[0003] Nowadays, optical information media are required to furtherincrease their information density in order to process a vast quantityof information as in images. The information density per unit area canbe increased either by narrowing the track pitch or by reducing thespace between recorded marks or between pits to increase a lineardensity. However, if the track density or linear density is too highrelative to the beam spot of reading light, the carrier-to-noise (C/N)ratio lowers, eventually to a level where signals are unreadable. Theresolution upon signal readout is determined by the diameter of a beamspot. More illustratively, provided that the reading light has awavelength λ and the optical system of the reading equipment has anumerical aperture NA, the spatial frequency 2NA/λ generally becomes aresolution limit. Accordingly, reducing the wavelength of reading lightand increasing the NA are effective means for improving the C/N andresolution upon readout. A number of technical studies that have beenmade thus far reveal that many technical problems must be solved beforesuch effective means can be introduced.

[0004] Under the circumstances, several methods have been proposed forgoing over the resolution limit (or diffraction limit) determined bylight diffraction. They are generally known as super-resolution readoutmethods.

[0005] The most common super-resolution readout method is to form a masklayer over a recording layer. Based on the fact that a laser beamdefines a spot having an intensity distribution approximate to theGaussian distribution, an optical aperture smaller than the beam spot isformed in the mask layer whereby the beam spot is reduced below thediffraction limit. This method is generally divided into a heat mode anda photon mode, depending on the optical aperture-forming mechanism.

[0006] The heat mode is such that upon irradiation to a beam spot, themask layer changes its optical properties in a region whose temperatureis raised above a certain value. The heat mode is utilized, for example,in the optical disc disclosed in JP-A 5-205314. This optical disc has ona transparent substrate in which optically readable recorded pits areformed in accordance with information signals, a layer of a materialwhose reflectance changes with temperature. That is, the material layerserves as a mask layer. The elements described in JP-A 5-205314 as thematerial of which the mask layer is constructed are lanthanoids, with Tbbeing used in Examples. In the optical disc of JP-A 5-205314, whenreading light is irradiated, the reflectance of the material layerchanges due to temperature distribution within the scanned spot of thereading light. After reading operation, the reflectance resumes theinitial state as the temperature lowers. It never happens that thematerial layer be melted during reading. Another known example of theheat mode is a medium capable of super-resolution readout, as disclosedin Japanese Patent No. 2,844,824, the medium having a mask layer of anamorphous-crystalline phase transition material in which ahigh-temperature region created within a beam spot is transformed intocrystal for increasing the reflectance. This medium, however, isimpractical in that after reading, the mask layer must be transformedback to amorphous.

[0007] The heat mode media require that the readout optical power bestrictly controlled in consideration of various conditions including thelinear velocity of the medium since the size of the optical aperturedepends solely on the temperature distribution in the mask layer. This,in turn, requires a complex control system and hence, an expensivemedium drive. The heat mode media also suffer from the problem thatreading characteristics degrade with the repetition of reading operationbecause the mask layer is prone to degradation by repetitive heating.

[0008] On the other hand, the photon mode is such that upon exposure toa beam spot, the mask layer changes its optical properties in a regionwhose photon intensity is increased above a certain value. The photonmode is utilized, for example, in the information recording medium ofJP-A 8-96412, the optical recording medium of JP-A 11-86342, and theoptical information recording medium of JP-A 10-340482. Moreillustratively, JP-A 8-96412 discloses a mask layer formed ofphthalocyanine or a derivative thereof dispersed in a resin or inorganicdielectric, and a mask layer formed of a chalcogenide. JP-A 11-86342uses as the mask layer a super-resolution readout film containing asemiconductor material having a forbidden band which upon exposure toreading light, is subject to electron excitation to the energy level ofexcitons to change light absorption characteristics. One illustrativemask layer is CdSe microparticulates dispersed in a SiO₂ matrix. JP-A10-340482 uses as the mask layer a glass layer in which the intensitydistribution of transmitted light varies non-linearly with the intensitydistribution of irradiated light.

[0009] Unlike the super-resolution readout media of the heat mode, thesuper-resolution readout media of the photon mode are relativelyresistant to degradation by repetitive reading.

[0010] In the photon mode, the region whose optical characteristicschange is determined by the number of incident photons which in turn,depends on the linear velocity of the medium relative to the beam spot.Also in the photon mode, the size of an optical aperture depends on thepower of reading light, indicating that supply of an excessive powermakes so large an optical aperture that super-resolution readout maybecome impossible. Therefore, the photon mode also requires to strictlycontrol the power of reading light in accordance with the linearvelocity and the size of pits or recorded marks (objects to be readout). Additionally, the photon mode requires to select the masklayer-forming material in accordance with the wavelength of readinglight. That is, the photon mode media are rather incompatible withmulti-wavelength reading.

[0011] Even in the case of recorded marks or pits of a large size whichdo not need super-resolution readout, a fully high C/N is not availableif the size is approximate to the resolution limit determined by lightdiffraction.

SUMMARY OF THE INVENTION

[0012] An object of the invention is to provide a readout method for anoptical information medium having pits or recorded marks of a sizeapproximate to the resolution limit determined by diffraction, whichmethod provides readings of the pits or recorded marks at a high C/N.

[0013] A first embodiment of the invention provides an informationreadout method for an optical information medium comprising aninformation recording layer having pits or recorded marks representativeof information data. The method includes the step of irradiating a laserbeam to the information recording layer through an objective lens forproviding readings of the pits or recorded marks. When the laser beamhaving a wavelength λ of 400 to 410 nm is irradiated through theobjective lens having a numerical aperture NA of 0.70 to 0.85 to thepits or recorded marks having a minimum size P_(L) of up to 0.36λ/NA,the laser beam is given a power Pr of at least 0.4 mW. The minimum sizeP_(L) is preferably up to 0.31λ/NA and also preferably, at least0.25λ/NA. The power Pr is preferably at least 0.45 mW and morepreferably at least 0.5 mW.

[0014] A second embodiment of the invention provides an informationreadout method for an optical information medium comprising aninformation recording layer having pits or recorded marks representativeof information data. The method includes the step of irradiating a laserbeam to the information recording layer through an objective lens forproviding readings of the pits or recorded marks. When the laser beamhaving a wavelength λ of 630 to 670 nm is irradiated through theobjective lens having a numerical aperture NA of 0.60 to 0.65 to thepits or recorded marks having a minimum size P_(L) of up to 0.36λ/NA,the laser beam is given a power Pr of at least 1.0 mW. The minimum sizeP_(L) is preferably up to 0.27λ/NA and also preferably at least0.25λ/NA. The power Pr is preferably at least 1.4 mW, more preferably atleast 2.0 mW, and even more preferably at least 2.2 mW.

[0015] Readout apparatus used in the information readout methods of thefirst and second embodiments are also contemplated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a fragmentary cross-sectional view of an opticalinformation medium according to one embodiment of the invention.

[0017]FIG. 2 schematically illustrates the operation of the invention.

[0018]FIG. 3 is a graph of C/N versus mark length.

[0019]FIG. 4 is a graph of C/N versus readout power.

[0020]FIG. 5 is a graph of C/N versus pit length.

[0021]FIG. 6 is a graph of C/N versus readout power.

[0022]FIG. 7 is a graph of C/N versus mark length.

[0023]FIG. 8 is a graph of C/N versus readout power.

[0024]FIG. 9 is a graph of C/N versus readout power.

[0025]FIG. 10 is a graph of C/N versus readout power.

[0026]FIG. 11 is a graph of C/N versus readout power.

[0027]FIG. 12 is a graph of C/N versus readout power.

[0028]FIG. 13 is a graph of C/N versus a tungsten content in Mo—W alloy.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] The information readout method of the invention is applied to anoptical information medium which includes a specific layer to bereferred to as “functional layer,” hereinafter. According to theinvention, pits or recorded marks are read out by irradiating a laserbeam thereto through a readout light irradiating optical system whilesetting the readout power Pr of the laser beam within a specific rangein accordance with the readout wavelength and the optical system. Thissetting enables super-resolution readout beyond the resolution limitdetermined by light diffraction. In addition, the invention achieves ahigher C/N ratio than in the prior art, when the pits or recorded markshaving a size approximate to and slightly greater than the resolutionlimit are read out.

[0030] In the first embodiment of the invention, the laser beam used inreading of pits or recorded marks (simply abbreviated as “pit/mark,”hereinafter) has a wavelength λ of 400 to 410 nm. The laser beam isirradiated to the medium through an objective lens having a numericalaperture NA of 0.70 to 0.85.

[0031] Since the cutoff spatial frequency is 2NA/λ, rows of pits/marksin which the pit/mark length is equal to the space between adjacentpits/marks are readable as long as the spatial frequency is equal to orbelow 2NA/λ (line pairs/nm). The pit/mark length (=space length)corresponding to the readable spatial frequency is given as

λ/4NA=0.25λ/NA.

[0032] It is then concluded that super-resolution readout is possible ifa C/N is obtained from a pit/mark row with a pit/mark length of lessthan 0.25λ/NA. Notably, the first embodiment is characterized in that ahigh C/N is available upon reading of pits/marks which are slightlylarger than the resolution limit. Then in the first embodiment, thepits/marks preferably have a minimum size P_(L) of at least 0.25λ/NA.Since too large a minimum size P_(L) of pits/marks compromises thebenefits of the first embodiment, the upper limit of the minimum sizeP_(L) is 0.36λ/NA, which is slightly larger than the resolution limit,and preferably up to 0.31λ/NA.

[0033] In the first embodiment, the laser beam irradiated for readoutshould have a power Pr of at least 0.4 mW, preferably at least 0.45 mW,and more preferably at least 0.5 mW. In the situation that thewavelength λ and the numerical aperture NA are within the limited rangesaccording to the first embodiment, when pits/marks having theabove-defined size approximate to the resolution limit corresponding tothe λ and NA are read out, a satisfactory C/N is available as long asthe readout power Pr is within the range limited by the firstembodiment.

[0034] In the second embodiment of the invention, the laser beam used inreading of pits/marks has a wavelength λ of 630 to 670 nm. The laserbeam is irradiated to the medium through an objective lens having anumerical aperture NA of 0.60 to 0.65.

[0035] The second embodiment is also characterized in that a high C/N isavailable upon reading of pits/marks which are slightly larger than theresolution limit. Then in the second embodiment, the pits/markspreferably have a minimum size P_(L) of at least 0.25λ/NA. Since toolarge a minimum size P_(L) of pits/marks compromises the benefits of thesecond embodiment, the upper limit of the minimum size P_(L) is0.36λ/NA, which is slightly larger than the resolution limit, andpreferably up to 0.27λ/NA.

[0036] In the second embodiment, the laser beam irradiated for readoutshould have a power Pr of at least 1.0 mW, preferably at least 1.4 mW,more preferably at least 2.0 mW, and even more preferably at least 2.2mW. In the situation that the wavelength λ and the numerical aperture NAare within the limited ranges according to the second embodiment, whenpits/marks having the above-defined size approximate to the resolutionlimit corresponding to the λ and NA are read out, a satisfactory C/N isavailable as long as the readout power Pr is within the range limited bythe second embodiment.

[0037] It is noted that in the first and second embodiments, noparticular upper limit is imposed on the readout power Pr. In general,the C/N becomes higher as the readout powder Pr increases. However, ifthe readout power Pr is high, the functional layer can be degraded byreadout operation or repetition thereof. With a high readout power Pr, areflected light detecting system of the medium readout apparatus can besaturated when the medium has a certain reflectance, disabling readoutoperation. These restraints impose a substantial upper limit on thereadout power Pr.

[0038] Although the reason why the C/N is significantly improved inproximity to the resolution limit by providing the functional layer isnot well understood, the inventors believe that the mechanism describedbelow participates therein.

[0039] The inventors presume as follows. When a laser beam is irradiatedto pits/marks, electric fields are created around the pits/marks asshown in FIG. 2. The intensity of the electric field or the extent towhich the electric field covers is correlated to the energy density onthe laser beam irradiated surface, that is, the energy per unit area ofthe irradiated surface and the material of which the irradiated surfaceis made. When the distance between adjacent pits/marks is relativelyshort, an interaction occurs between the electric fields of adjacentpits/marks, which enables super-resolution readout. This electric fieldis likely to generate near an edge when a structure has an edge likepits. The electric field is also likely to generate at the site wheredielectric constant or electric conductivity sharply changes, such asthe boundary between crystalline and amorphous phases, like the outerperiphery of amorphous recorded marks in phase change optical recordingmedia.

[0040] The interaction depends on the energy density of the laser beam.Then the interaction becomes stronger in proportion to the readout powerPr, if the readout wavelength λ and the numerical aperture NA of thereadout optical system are the same. As a consequence, the C/N becomeshigher as the readout power Pr increases. Besides, the laser beam spotdiameter is in proportion to λ/NA, which indicates that with the readoutpower Pr kept fixed, the C/N becomes higher as the readout wavelength λis shorter and as the numerical aperture NA is larger. Inversely, ifλ/NA is small, an equivalent C/N is obtainable with a lower readoutpower Pr.

[0041] On the other hand, when the distance between adjacent pits/marksis relatively long, the above-described interaction does not occur, oronly a little interaction occurs if any, or the signal componentproduced by the interaction is small as compared with the signalcomponent produced by ordinary readout operation. As a result, the C/Nimprovement dependent on the readout power Pr is not ascertained or isextremely little if any.

[0042] It is noted that for a medium having a mask layer (described inthe preamble), as is evident from the operation principle of the masklayer, effects are achieved only when pits or recorded marks, which areobjects to be read out, have a small size below the resolution limit.The medium fails to produce a higher C/N than the prior art masklayer-free media when the objects to be read out have a size slightlylarger than the resolution limit.

[0043] The present invention is of great worth in the industry. Opticaldiscs featuring the highest recording density among the currentlyavailable optical discs are DVD. DVD-RW, which is DVD for Re-RecordableDisc, has a phase change recording layer, which functions as thefunctional layer. For the readout operation of DVD-RW, the readoutwavelength λ and numerical aperture NA fall within the second embodimentof the invention. Therefore, the application of the second embodiment toDVD-RW enables super-resolution readout and significantly improves theC/N in proximity to the resolution limit.

[0044] The inventors carried out an experiment of applying the secondembodiment of the invention to a commercially available DVD-RW disc(manufactured by TDK Corporation). In this experiment, the readoutwavelength λ was 650 nm, and the objective lens of the readout opticalsystem had a numerical aperture NA of 0.60 according to the DVD-RWstandards. The results are shown in FIG. 3. In FIG. 3, when the readoutpower Pr is 1 mW, a C/N of about 20 to 30 dB is obtained at a marklength of 0.28 to 0.3 μm which is approximate to the resolution limit(0.271 μm). Also in FIG. 3, when the readout power Pr is 2.4 mW, a C/Nof about 40 dB is obtained at a mark length of 0.28 to 0.3 μm which isapproximate to the resolution limit, indicating that fully practicalreadout is possible. Namely, the present invention permits aconventional optical disc to achieve a substantial C/N improvement inproximity to the resolution limit simply by increasing the readout powerPr.

[0045] In the disclosure, the medium is considered readable (readout ispossible) when a C/N of at least 20 dB is obtained. For the medium tofind practical use, a C/N of preferably at least about 30 dB and morepreferably at least about 40 dB is necessary.

[0046]FIG. 4 is a graph of C/N versus readout power Pr when recordedmarks of different length are read out in the above-described DVD-RWdisc. It is evident from FIG. 4 that the C/N improves in proximity tothe resolution limit as the readout power Pr increases.

[0047] The inventors also carried out an experiment of applying thesecond embodiment of the invention to a read-only memory (ROM) typedisc. This ROM disc includes a resin substrate on which pits having alength shown in FIG. 5 are arrayed in rows, a Ge layer of 15 nm thickformed thereon as a reflecting and functional layer, and a protectivelayer formed thereon of a UV-curable resin to a thickness of 10 μm. Inthe pit row, the space between adjacent pits is equal to the length ofpits.

[0048] For C/N measurement, readout operation was carried out on the ROMdisc using a laser beam having a readout wavelength λ of 635 nm and theobjective lens of the readout optical system having a numerical apertureNA of 0.60. The results are shown in FIG. 5. Under these readoutconditions, pits having a length of 0.3 μm are, for the most part,readable, but their size is approximate to the resolution limit (0.265μm), and pits having a length of 0.25 μm or less are, for the most part,unreadable. FIG. 5 shows C/N as a function of pit length when thereadout power is 1 mW or 5 mW. It is seen from FIG. 5 that at a pitlength of 0.4 μm or more, approximately equal C/N is obtainable fromboth the readout powers, but at a pit length of 0.3 μm or less, the C/Nincreases as the readout power rises. More specifically, when pitshaving a length of 0.2 to 0.3 μm are to be read out, the readout powerof 1 mW provides a C/N of less than 40 dB, and the readout power of 5 mWprovides a C/N of more than 40 dB, indicating that fully practicalreadout is possible.

[0049]FIG. 6 shows C/N as a function of a readout power when the pitlength is 0.3 μm, 0.25 μm or 0.2 μm. It is clearly seen from FIG. 6 thatwhen pits having a size which is generally readable and approximate tothe resolution limit are to be read out, the C/N improves in proportionto the increasing readout power.

[0050]FIGS. 7 and 8 show the results of an experiment demonstrating thebenefits of the first embodiment of the invention. This experiment usedan optical disc having a phase change recording layer, a readoutwavelength λ of 405 nm and a numerical aperture NA of 0.85.

[0051]FIG. 7 is a graph of C/N versus mark length when the readout powerPr is 0.5 mW in accordance with the first embodiment of the inventionand 0.3 mW outside the scope of the invention. FIG. 8 is a graph of C/Nversus readout power Pr when recorded marks of different length are readout. FIGS. 7 and 8 demonstrate the benefits of the invention, and it isalso evident from FIGS. 7 and 8 that in proximity to the resolutionlimit (0.119 μm), the C/N improves as the readout power Pr rises.

[0052] It is noted that the optical disc used in the experiment theresults of which are shown in FIGS. 7 and 8 includes a reflective layer,a second dielectric layer, a recording layer, a first dielectric layerand a light transmitting layer stacked in order on a supportingsubstrate. Recording/reading light enters the recording layer throughthe light transmitting layer. The supporting substrate used was apolycarbonate disc having a diameter of 120 mm and a thickness of 1.2 mmin which grooves are formed at the same time as injection molding of thedisc itself. The reflective layer was 100 nm thick and formed ofAg₉₈Pd₁Cu₁. The second dielectric layer was 20 nm thick and formed ofAl₂O₃. The recording layer was 12 nm thick and had the composition(atomic ratio) of In_(1.1)Sb_(74.6)Te_(18.6)Ge_(5.7). The firstdielectric layer was 130 nm thick and formed of 80 mol % ZnS and 20 mol% SiO₂. The light transmitting layer was 100 μm thick and formed by spincoating a UV-curable resin, followed by UV curing.

[0053] As the benefits of the invention are accomplished with not onlyrecording media, but also read-only media according to the secondembodiment using the readout optical system having a large λ/NA value,the benefits of the invention are also accomplished with read-only mediaaccording to the first embodiment using the readout optical systemhaving a small λ/NA value.

[0054] The optical information medium according to the invention has aninformation recording layer. The information recording layer used hereinis a layer that has projections and depressions in the form of pitsand/or grooves, a layer where recorded marks can be formed, or a layerthat has projections and depressions and can form recorded marks. Thissuggests that the invention is applicable to both read-only media andoptical recording media (write-once or rewritable media). In theread-only media, a reflective layer covering pits formed in a substratesurface and composed of a metal, metalloid or compound constitutes theinformation recording layer. In the optical recording media, therecording layer constitutes the information recording layer. Therecording layer may be any of a phase change layer, a layer based on anorganic dye, and a layer based on another organic material or inorganicmaterial. The recorded marks may take the form of areas having adifferent optical constant (e.g., reflectance) from the surrounding,concave areas or convex areas.

[0055] We have found that by providing an optical information mediumwith a layer constructed of a specific material and having anappropriate thickness for the specific material, the optical informationmedium is given a capability of super-resolution readout based on amechanism essentially different from the prior art and that pits orrecorded marks having a size somewhat larger than the resolution limitcan be read out at a significantly higher C/N than in the prior art. Thespecific material used herein is at least one element selected fromamong Nb, Mo, W, Mn, Pt, C, Si, Ge, Ti, Zr, V, Cr, Fe, Co, Ni, Pd, Sb,Ta, Al, In, Cu, Sn, Te, Zn, and Bi or an alloy or compound containingthat element or elements, with the element or compound thereof beingpreferred. Herein, the layer capable of super-resolution readout isdesignated a functional layer. The provision of the functional layerenables to detect pits, grooves or recorded marks of a size fallingbelow the resolution limit determined by light diffraction. Aspreviously described, the information recording layer can be utilized asthe functional layer in the practice of the invention.

[0056] Application to Medium Structure of FIG. 1

[0057] Referring to FIG. 1, there is illustrated one exemplaryconstruction of the optical information medium. The optical informationmedium 1 shown in FIG. 1 is a read-only medium including alight-transmissive substrate 2 having pits 21 formed on a surface and alayer 10 in close contact with the pitted surface. Reading light comesfrom below in the figure. The layer 10 serves as the functional layerwhen it has a specific composition and a specific thickness.

[0058] Layer 10 Made of Element or Alloy

[0059] Optical disc samples of the structure shown in FIG. 1 werefabricated by the following procedure. The substrate 2 used was a discwhich was injection molded from polycarbonate (refractive index n=1.58)to a diameter of 120 mm and a thickness of 0.6 mm in which phase pitswere formed simultaneous with injection molding. The substrate 2 is ofthe banded type that the substrate is provided with a plurality ofannular pit-forming regions having helical tracks in a concentricpattern, and pits have an equal length in each pit-forming region.Namely, phase pits of different lengths are formed in a singlesubstrate. The pit length in each pit-forming region is 250 nm. Thespace between adjacent pits is equal to the pit length. The layer 10 wasconstructed of any one element selected from among Nb, Mo, W, Mn, Pt, C,Si, Ge, Ti, Zr, V, Cr, Fe, Co, Ni, Pd, Sb, Ta, Al, In, Cu, Sn, Te, Zn,Bi, Au and Ag and has a thickness of 5 to 100 nm. The layer 10 wasformed by sputtering.

[0060] Using an optical disc tester (laser wavelength 635 nm andnumerical aperture 0.60), these samples were measured for carrier tonoise (C/N) ratio at a linear velocity of 11 m/s and a readout powerchanging in the range of 1 to 7 mW. Since the optical disc tester usedhas a cutoff spatial frequency 2NA/λ, which is calculated to be:

2NA/λ=1.89×10³ (line pairs/mm),

[0061] rows of pits in which the pit length is equal to the spacebetween adjacent pits are readable as long as the spatial frequency isequal to or below 1.89×10³ (line pairs/mm). The pit length (=spacelength) P_(L) corresponding to the readable spatial frequency is givenas

P _(L)λ/4NA=265 (nm).

[0062] It is then concluded that super-resolution readout is possible ifa C/N is obtained from a pit row with a pit length of less than 265 nm.

[0063] Tables 1 to 4 show the relationship of C/N to the thickness ofthe layer 10. In Tables 1 to 4, the highest C/N obtained from the layer10 of a certain thickness when the readout power is changed between 1 mWand 7 mW is reported for each of different materials of which the layer10 is made. Table 1 picks up those samples which show a maximum C/N ofat least 40 dB, Table 2 picks up those samples which show a maximum C/Nof 30 dB to less than 40 dB, Table 3 picks up those samples which show amaximum C/N of 20 dB to less than 30 dB, and Table 4 picks up thosesamples which show a maximum C/N of less than 20 dB. TABLE 1 C/N (dB)versus thickness of layer 10 (maximum C/N ≧ 40 dB) Layer 10 Thickness oflayer 10 (nm) Material 5 10 15 20 30 50 100 Nb 38.4 — 34.3 — 44.1 40.332.8 Mo — 41.2 43.0 — 39.6 26.4 9.2 W 32.2 43.0 43.6 38.2 32.7 24.3 7.7Mn 33.2 37.6 35.3 — 40.7 35.1 22.7 Pt — 39.1 40.2 — 30.2 13.2 4.3 C 33.2— — — 40.9 40.9 31.0 Si 45.5 43.2 47.1 — 41.4 44.9 40.5 Ge 37.4 41.345.0 — 44.4 42.5 40.7

[0064] TABLE 2 C/N (dB) versus thickness of layer 10 (40 dB > maximumC/N ≧ 30 dB) Layer 10 Thickness of layer 10 (nm) Material 5 10 15 20 3050 100 Ti — 29.6 35.4 37.2 37.5 37.4 29.8 Zr — — 20.9 — — 36.7 28.8 V33.1 — 31.1 — 36.6 39.6 32.4 Cr — 35.2 26.8 — 20.4 11.1 4.8 Fe 28.6 29.535.8 — 35.2 29.4 7.9 Co — 31.8 37.0 39.4 35.9 26.3 6.2 Ni — — 36.3 —37.1 28.8 5.0 Pd — 32.8 38.0 — 31.4 14.5 5.4 Sb — 29.4 35.6 — 36.1 33.323.7 Ta — 23.3 25.0 — 31.5 33.8 21.5 Al — 34.9 26.4 —  0.0  0.0 0.0 In33.6 24.2 21.3 — 27.9 25.9 22.2

[0065] TABLE 3 C/N (dB) versus thickness of layer 10 (30 dB > maximumC/N ≧ 20 dB) Layer 10 Thickness of layer 10 (nm) Material 5 10 15 20 3050 100 Cu — 21.9 0.0 — 7.6 9.3 8.6 Sn — 25.5 26.9 — 21.0 9.9 3.7 Te 28.023.6 25.9 — 27.0 24.0 18.0 Zn  0.0 0.0 12.8 — 0.0 29.0 10.6 Bi  0.0 0.00.0 — 13.0 23.7 11.4

[0066] TABLE 4 C/N (dB) versus thickness of layer 10 (maximum C/N < 20dB) Layer 10 Thickness of layer 10 (nm) Material 5 10 15 20 30 50 100 Ag19.2 7.4 7.8 — 0.0 0.0 0.0 Au 12.2 8.9 5.6 4.9 8.3 5.5 7.1

[0067] It is evident from Tables 1 to 4 that the thickness of the layer10 must be optimized for a particular element used in order thatsuper-resolution readout become possible. For example, as seen fromTable 2, super-resolution readout is possible when the layer 10 is an Allayer and has a thickness of 15 nm. However, when the Al layer has athickness of 100 nm, which is approximate to the thickness of thereflective layer in conventional ROM discs such as CD-ROM and DVD-ROM,super-resolution readout becomes impossible like conventional ROM discs.

[0068] Only for those samples producing a maximum C/N among the abovesamples, FIGS. 9 to 12 show C/N relative to readout power Pr. Thesamples shown in FIGS. 9 to 12 correspond to the samples shown in Tables1 to 4, respectively. The C/N was measured on a pit row with a pitlength of 250 nm. The C/N measurement used the same optical disc testeras above and a linear velocity of 11 m/s. It is seen from FIGS. 9 to 12that in most samples, C/N increases with an increasing readout power.Although the signal intensity is not shown in these diagrams, the signalintensity shows the same behavior as the C/N. In FIGS. 9 to 12, thesamples lacking data on the high Pr region are those samples whichfailed to produce readout signals on account of degradation of the layer10 at such Pr or mean that no data were available due to saturation ofthe reflected light detecting system of the tester.

[0069] Further, a sample having the layer 10 made of atungsten-molybdenum (W—Mo) alloy having a thickness of 15 nm wasmeasured for C/N along a pit row with a pit length of 250 nm using thesame optical disc tester as above at a linear velocity of 11 m/s. Theresults are shown in FIG. 13. It is seen from FIG. 13 thatsuper-resolution readout is also possible when an alloy is used.

[0070] It is noted that although the above experiments evaluated C/N forthose media having rows of pits whose size was smaller than theresolution limit, similar effects were accomplished for those mediahaving rows of pits whose size was equal to or greater than theresolution limit and equal to or less than 0.31λ/NA. Namely, when thethickness of the layer 10 is properly set in accordance with theparticular material thereof, a C/N improvement over media having analuminum layer of 100 nm thick is observable.

[0071] Layer 10 Made of Compound

[0072] Even when the layer 10 is constructed of various compounds suchas nitrides, oxides, fluorides, sulfides, and carbides, the opticalinformation medium of the invention is also capable of super-resolutionreadout and capable of improving C/N in reading out objects having asize slightly larger than the resolution limit, and intrinsic effectsare exerted. It is noted that the compounds used herein are not limitedto stoichiometric compounds and encompass mixtures of metals ormetalloids with nitrogen, oxygen, etc. in a proportion less than thestoichiometric composition. Namely, the layer 10 falls within the scopeof the invention that contains a metal or metalloid which is capable ofsuper-resolution readout when used in an elemental or alloy form andadditionally, another element, preferably at least one element selectedfrom among nitrogen, oxygen, fluorine, sulfur and carbon. Theconstruction of the layer 10 from such compounds is effective forspreading the readout power margin, improving the C/N, and suppressingdegradation of C/N by repetitive reading. For the layer 10 constructedof compounds, operation and results are described below.

[0073] First, the improvement in chemical stability of the layer 10 dueto compound formation is described together with the concomitantadvantages. In general, metals excluding noble metals (e.g., Au) ormetalloids naturally produce in the form of compounds such as oxides andsulfides. This fact indicates that in the ordinary environment, metalsor metalloids are more stable when present in compound form than in pureelemental form. That is, metals or metalloids are significantly improvedin chemical stability by converting them into compounds. On the otherhand, the degradation of the layer 10 by high power reading andrepetitive reading is presumably due to a chemical change (typicallyoxidation) caused by a temperature rise of the layer 10. Since the layer10 is in contact with air, it is susceptible to degradation by heatingduring application of readout power. However, if the layer 10 is formedof a compound, it is restrained from a chemical change. Then, readingbecomes possible with a higher power, the maximum C/N is improved, andthe degradation of C/N by repetitive reading is restrained. Therefore,the formation of the layer 10 from a compound is quite effective when amaterial which undergoes degradation at a relatively low readout poweris used.

[0074] Next, the increase in transparency of the layer 10 due tocompound formation is described together with the concomitantadvantages. When the layer 10 is formed of a compound, its transparencyincreases, and its optical reflectance lowers accordingly. When thelayer 10 is reduced in optical reflectance, it becomes unlikely that thereflected light detecting system is saturated. This results in anincrease of the permissible readout power and hence an improvement inmaximum C/N. Since the layer 10 formed of a compound is improved intransparency per unit thickness, the layer 10 of a compound avoidssaturation of the reflected light detecting system even when the layer10 is made thicker. For this reason, the thickness range of the layer 10within which super-resolution readout is possible is significantlyexpanded. Therefore, the formation of the layer 10 from a compound isquite effective when a material which causes the reflected lightdetecting system to be saturated at a relatively low readout power isused.

[0075] In order that the layer 10 be formed of a compound, use ispreferably made of a reactive sputtering technique using a reactive gassuch as nitrogen or oxygen or a sputtering technique using a compoundtarget. Other techniques such as CVD may also be utilized.

[0076] Thickness of Layer 10

[0077] As seen from the results of the above-described experiments, thelayer which is constructed of an elemental metal or metalloid shouldpreferably have the following thickness, which is given for therespective elements.

[0078] Nb: up to 100 nm

[0079] Mo: up to 70 nm, especially up to 45 nm

[0080] W: up to 70 nm, especially up to 40 nm

[0081] Mn: up to 100 nm, especially up to 70 nm

[0082] Pt: up to 40 nm, especially up to 30 nm

[0083] C: up to 100 nm

[0084] Si: up to 100 nm

[0085] Ge: up to 100 nm

[0086] Ti: up to 100 nm

[0087] Zr: up to 100 nm, especially 25 to 100 nm

[0088] V: up to 100 nm

[0089] Cr: up to 30 nm, especially less than 15 nm

[0090] Fe: up to 80 nm, especially up to 50 nm

[0091] Co: up to 70 nm, especially up to 45 nm

[0092] Ni: up to 70 nm, especially up to 50 nm

[0093] Pd: up to 40 nm, especially up to 30 nm

[0094] Sb: up to 100 nm, especially up to 60 nm

[0095] Ta: up to 100 nm, especially up to 60 nm

[0096] Al: up to 20 nm, especially less than 15 nm

[0097] In: up to 100 nm, especially less than 10 nm

[0098] Cu: up to 10 nm

[0099] Sn: up to 40 nm

[0100] Te: up to 70 nm

[0101] Zn: 40 to 90 nm

[0102] Bi: 25 to 70 nm

[0103] It is noted that for those elements which produce asatisfactorily high C/N even at a thickness of 100 nm, the thicknessupper limit of 100 nm need not be set from the performance standpoint,but limiting the thickness to 100 nm or less is preferred for preventinga productivity drop. Also preferably, the layer 10 should have athickness of at least 2 nm regardless of the element of which the layeris made. If the layer 10 is too thin, the reflectance may become too lowfor the tracking servo system to perform well, failing to produce asatisfactory C/N.

[0104] When the layer 10 is formed of compounds, the appropriatethickness range of the layer 10 is expanded as described above.

[0105] Now the functional layer constructed by an alloy is described. Bythe term “functional element” used below is meant an element which alonecan construct the functional layer.

[0106] When the functional layer is constructed by a binary alloy in thesimple solid solution form as typified by the above-described W—Mo alloywherein both the elements are functional elements, the alloy layerserves as the functional layer as seen from FIG. 13.

[0107] For an alloy layer in the simple solid solution form, it isdesired that at least one, preferably all of the constituent elements befunctional elements. The molar proportion of functional elements ispreferably at least 50% of the entire constituent elements.

[0108] Like the alloy layer in the simple solid solution form, it isdesired for an amorphous alloy layer such as a magneto-optical recordingmaterial layer that at least one, preferably all of the constituentelements be functional elements. The molar proportion of functionalelements is preferably at least 50% of the entire constituent elements.

[0109] Ag—In—Sb—Te base phase change materials are phase separation typealloys in which the Sb phase separates from other phases uponcrystallization. For such a phase separation type alloy, it is desiredthat at least one, preferably all of the constituent phases canconstruct a functional layer alone. For example, the Sb phase in acrystallized Ag—In—Sb—Te alloy serves as a functional layer alone.

[0110] Like the single element layer, the alloy layer must satisfy thethickness requirement in order to serve as the functional layer. Forexample, the alloy layer in the simple solid solution form may be set toa sufficient thickness for a single element layer of each functionalelement to serve as the functional layer, as shown in FIG. 13.

[0111] The specific composition and thickness of an alloy layer arepreferably determined only after it is actually inspected whether analloy layer having a particular composition and thickness serves as thefunctional layer. For example, intermetallic compounds such as theabove-described phase change material of Ge₂Sb₂Te₅ often exhibit abehavior unexpected from the respective constituent elements alone.

[0112] Reading Method

[0113] In the medium of the invention, an upper limit is imposed on theapplicable readout power, depending on the material of the layer 10 andthe structure of the medium. It is therefore convenient that an optimumreadout power for these conditions is previously recorded in the mediumof the invention. Then, the optimum readout power can be read out beforethe start of reading operation, and the reading operation be performedwith this optimum power. Also, if necessary, a trial reading operationmay be performed to determine the optimum readout power.

[0114] Japanese Patent Application Nos. 2001-123521 and 2002-093026 areincorporated herein by reference.

[0115] Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. An information readout method for an optical information medium comprising an information recording layer having pits or recorded marks representative of information data, said method comprising the step of irradiating a laser beam to the information recording layer through an objective lens for providing readings of the pits or recorded marks, wherein when the laser beam having a wavelength λ of 400 to 410 nm is irradiated through the objective lens having a numerical aperture NA of 0.70 to 0.85 to the pits or recorded marks having a minimum size P_(L) of up to 0.36λ/NA, the laser beam is given a power Pr of at least 0.4 mW.
 2. The information readout method of claim 1 wherein the minimum size P_(L) is up to 0.31λ/NA.
 3. The information readout method of claim 1 wherein the minimum size P_(L) is at least 0.25λ/NA.
 4. The information readout method of claim 1 wherein the power Pr is at least 0.45 mW.
 5. The information readout method of claim 1 wherein the power Pr is at least 0.5 mW.
 6. An information readout method for an optical information medium comprising an information recording layer having pits or recorded marks representative of information data, said method comprising the step of irradiating a laser beam to the information recording layer through an objective lens for providing readings of the pits or recorded marks, wherein when the laser beam having a wavelength λ of 630 to 670 nm is irradiated through the objective lens having a numerical aperture NA of 0.60 to 0.65 to the pits or recorded marks having a minimum size P_(L) of up to 0.36λ/NA, the laser beam is given a power Pr of at least 1.0 mW.
 7. The information readout method of claim 6 wherein the minimum size P_(L) is up to 0.27λ/NA.
 8. The information readout method of claim 6 wherein the minimum size P_(L) is at least 0.25λ/NA.
 9. The information readout method of claim 6 wherein the power Pr is at least 1.4 mW.
 10. The information readout method of claim 6 wherein the power Pr is at least 2.0 mW.
 11. The information readout method of claim 6 wherein the power Pr is at least 2.2 mW.
 12. A readout apparatus used in the information readout method of claim
 1. 13. A readout apparatus used in the information readout method of claim
 6. 